1. Field of Invention
The present invention relates to an electronic programmable wall switch timer for use in lieu of an ordinary wall switch, thereby to provide for programmable control of the power provided to the load normally controlled from this wall switch.
2. Prior Art
A couple of types of electronic programmable wall switch timers are available for purchase, and many more types have been described in various publications. The most commonly used type is described in U.S. Pat. No. RE 31,848 to Nilssen.
However, each one of these electronic wall switch timers has at least two significantly limiting characteristics: i) the use a thyristor for controlling the flow of power, and ii) the use of the 60 Hz power line voltage for providing the requisite accurate time base for the programming means.
One limitation associated with the use of a thyristor in a wall switch timer relates to one of the thyristor's likely failure modes, namely the one in which the thyristor changes to become a rectifier. This likely failure mode makes it is hard to obtain U.L. listing for a thyristor-based wall switch timer to be used for loads other than incandescent lamps.
Another limitation associated with the use of a thyristor involves inefficiency: because of the relatively large amount of power dissipation occurring within a thyristor, it is necessary that it be used with a heat sink; which results in significant limitations of the design freedom associated with any timer using a thyristor for power switching.
The main limitation associated with using the 60 Hz power line voltage as the basis for keeping accurate track of time is that of losing timing and/or accuracy of programming as a result of load failure or removal, or due to power failure.